The invention relates to thin film batteries and in particular to improved electrolytes for thin film batteries.
Thin-film rechargeable batteries have numerous applications in the field of microelectronics. For example, thin-film batteries provide active or standby power for microelectronic devices and circuits. Active power sources of the thin-film battery type are used, for example, in implantable medical devices, remote sensors, miniature transmitters, smart cards, and MEMS devices. Standby power sources of the thin-film battery type are used, for example, in PCMCIA cards and other types of CMOS-SRAM memory devices.
In a thin-film battery, a chemical reaction takes place between an anode and cathode by interaction of the anode and cathode through an electrolyte. The attractiveness of thin-film batteries over conventional batteries is that the electrolyte is a substantially solid or non-flowable material rather than a liquid. Liquid electrolytes pose leakage problems and are often highly corrosive. Of the solid electrolytes, thin-film batteries typically employ organic and ceramic electrolytes. Solid electrolytes are desirable in cells or batteries where liquid electrolytes may be undesirable, such as in implantable medical devices. Preferred solid electrolytes include materials that are solid at room temperature, electrically insulative and ionically conductive.
Examples of solid electrolytes include metallic salts and vitreous solid compositions. Metallic salt solid electrolytes include, for example, compounds that conform to the formula: AgIxe2x80x94MCNxe2x80x94AgCN, wherein M is potassium, rubidium, cesium or mixtures thereof. Vitreous solid compositions, or glasses, are generally comprised of a network former, a network modifier and a network dopant. A network former provides a macromolecular network of irregular structure. A network modifier is an ionic compound that becomes incorporated into the macromolecular network of the network former. A network dopant provides mobile cations to the network. While solid electrolytes are preferred for various applications, such solid electrolytes tend to exhibit lower specific conductivities than liquid electrolytes. Hence batteries employing solid electrolytes tend to operate at lower currents than batteries using liquid electrolytes.
As advances are made in microelectronic devices, new uses for thin-film batteries continue to emerge. Along with the new uses, there is a need for high performance thin-film batteries having improved properties such as higher electrolyte conductivities, more stable electrolytes, and the like. In particular, there is a need for thin film batteries that use solid electrolytes and that operate at higher currents as compared to current solid electrolyte batteries.
In one embodiment, with regard to the foregoing and other needs, the invention provides a solid amorphous electrolyte composition for a thin-film battery. The electrolyte composition includes a lithium phosphorus oxynitride material containing a sulfide ion dopant wherein the atomic ratio of sulfide ion to phosphorus ion (S/P) in the electrolyte ranges from greater than 0 up to about 0.2. The composition is represented by the formula:
LiwPOxNySz,
where 2x+3y+2z=5+w, x ranges from about 3.2 to about 3.8, y ranges from about 0.13 to about 0.46, z ranges from greater than zero up to about 0.2, and w ranges from about 2.9 to about 3.3.
In another embodiment the invention provides a method for making a solid electrolyte for a thin-film battery. The method includes the steps of:
providing a lithium orthophosphate (Li3PO4) composition;
providing a lithium ion and sulfide ion containing component selected from the group consisting of Li2SO4 and Li2S;
combining the lithium orthophosphate composition and the lithium ion and sulfide ion containing component to yield a sputtering target;
sputtering the target in a gas atmosphere selected from nitrogen gas, argon gas, and mixtures of nitrogen and argon gases to provide an electrolyte film having a composition represented by the formula:
LiwPOxNySz,
where 2x+3y+2z=5+w, x ranges from about 3.2 to about 3.8, y ranges from about 0.13 to about 0.46, z ranges from greater than zero up to about 0.2, and w ranges from about 2.9 to about 3.3, and wherein the ratio of sulfide ion to phosphorus ion (S/P) ranges from greater than 0 up to about 0.2.
In yet another embodiment, the invention provides a method for making a sulfide-doped lithium phosphorus oxynitride solid electrolyte for a thin-film battery. The method includes the steps of:
providing a lithium orthophosphate (Li3PO4) composition as a sputtering target;
sputtering the target in an atmosphere containing nitrogen gas and hydrogen sulfide gas wherein the mixture of nitrogen gas and hydrogen sulfide gas in the atmosphere is represented by the following:
(1xe2x88x92t)N2+tH2S,
where t is greater than 0 and less than 1 to provide an electrolyte film having a composition represented by the formula:
LiwPOxNySz,
where 2x+3y+2z=5+w, x ranges from about 3.2 to about 3.8, y ranges from about 0.13 to about 0.46, z ranges from greater than zero up to about 0.2, and w ranges from about 2.9 to about 3.3, and wherein the ratio of sulfide ion to phosphorus ion (S/P) ranges from greater than 0 up to about 0.2.
An important advantage of the invention is that thin-film batteries containing the electrolyte of the invention are capable of delivering more power and energy than thin-film batteries containing conventional undoped lithium phosphorus oxynitride (LIPON) electrolytes. By selecting a desired amount of sulfide doping, the lithium ion conductivity of the electrolyte of the invention can be significantly increased over the lithium ion conductivity of conventional LIPON electrolytes. At room temperature, a LIPON electrolyte has a lithium ion conductivity of about 2 xcexcS/cm and a transport number of unity. Because its conductivity is relatively low, the LIPON electrolyte is a dominant contributor to cell resistance. For example, in thin-film batteries containing a lithium cobalt oxide (LiCoO2) cathode, the LIPON electrolyte dominates the cell resistance until the thickness of the cathode reaches about 3.5 xcexcm. The invention enables an increase of the ionic conductivity of the electrolyte, resulting in an increase of the energy availability at higher discharge rates (higher powers) as compared to conventional LIPON electrolytes.